Hi C.J.,
Welcome to WAGS!
I always look for 100% sodium hydroxide for cleaning iron. Some drain cleaning products contain certain other chemicals, and I have always been told to avoid those, as they may not be food safe, like the lye most certainly is.
Your posts have made me curious about this, so I have looked up some info on corrosion of common metals by chemicals. Potassium hydroxide is chemically very similar to sodium hydroxide, the biggest difference being that it is more expensive. If I had a big ol bag of the stuff sitting around, I would probably want to try it out, as you are.
It does not react much at all with carbon steel, but it will corrode cast iron to some extent(more so than the sodium hydroxide). For this reason, I would limit experimentation to items that I did not have an emotional attachment to, and I would not leave the iron in the container any longer than necessary.
One of the best things about using sodium based lye is that the items can be left for an extended period, months or even years. If you have or could find some totally worthless item, cracked, warped, Chinese, whatever, and see what a prolonged bath will do.
I will be following this thread with great interest.
Chris